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%I #43 Apr 12 2013 13:13:21
%S 1,4,1276900,7236100,9449476,69529779225,273137935876,275693254225,
%T 1011814692100,1590221881600,3007619594001,5382738725329,
%U 6343774129225,10830009646404,43037339281225,49597341481444,161977776248401,492275260674729,623724701219361
%N Square numbers n for which sigma(n) - d(n) is also a perfect square.
%e 4 is in the list since 4 = 2^2 and sigma(4)-d(4) = 4 = 2^2. Also 9449476 = 3074^2 and sigma(9449476)-d(9449476) = 17455684 = 4178^2.
%t Sqd[n_] := Sqrt[DivisorSigma[1, n] - DivisorSigma[0, n]]; t = {}; Do[p = n^2; If[IntegerQ[Sqd[p]], AppendTo[t, p]], {n, 7000000}]; t
%Y Cf. A221856.
%K nonn
%O 1,2
%A _Jayanta Basu_, Apr 11 2013
%E a(16)-a(19) from _Donovan Johnson_, Apr 11 2013
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